Double clicking in general seems to be rather random in WOM.

When loading a save game I can click up to 10 times before it registers my intention to double click that save game slot.

The reverse is true as well.  Double clicking Raise Land can result in a Triple Click.

If there is a piece of land under the pointer position, that is also in spell range, the 2nd click goes through the open spell book and the spell immediately affects the piece of land under the pointer.
That you usually get to choose a target is because the spell range is short and the "applicable" land tiles are usually not under the mouse pointer when you double click the spell in the upper left of the book.

This will be far more apparent when more global / strategic spells come online with the magic revamp.